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>10.2. Logins via the network</A
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><P
>Two computers in the same network are usually linked via a
	single physical cable.	When they communicate over the network,
	the programs in each computer that take part in the communication
	are linked via a <I
CLASS="GLOSSTERM"
>virtual connection</I
>, a sort
	of imaginary cable.  As far as the programs at either end of the
	virtual connection are concerned, they have a monopoly on their
	own cable.  However, since the cable is not real, only imaginary,
	the operating systems of both computers can have several virtual
	connections share the same physical cable.  This way, using just
	a single cable, several programs can communicate without having
	to know of or care about the other communications.  It is even
	possible to have several computers use the same cable; the virtual
	connections exist between two computers, and the other computers
	ignore those connections that they don't take part in.	</P
><P
> That's a complicated and over-abstracted description of
	the reality.  It might, however, be good enough to understand
	the important reason why network logins are somewhat different
	from normal logins.  The virtual connections are established
	when there are two programs on different computers that wish
	to communicate.  Since it is in principle possible to login
	from any computer in a network to any other computer, there is
	a huge number of potential virtual communications.  Because of
	this, it is not practical to start a <B
CLASS="COMMAND"
>getty</B
>
	for each potential login.  </P
><P
> There is a single process inetd (corresponding to
	<B
CLASS="COMMAND"
>getty</B
>) that handles all network logins.
	When it notices an incoming network login (i.e., it notices
	that it gets a new virtual connection to some other computer),
	it starts a new process to handle that single login.  The original
	process remains and continues to listen for new logins.  </P
><P
> To make things a bit more complicated, there is
	more than one communication protocol for network logins.
	The two most important ones are <B
CLASS="COMMAND"
>telnet</B
> and
	<B
CLASS="COMMAND"
>rlogin</B
>.  In addition to logins, there are many
	other virtual connections that may be made (for FTP, Gopher, HTTP,
	and other network services).  It would be ineffective to have a
	separate process listening for a particular type of connection,
	so instead there is only one listener that can recognise the type
	of the connection and can start the correct type of program to
	provide the service.  This single listener is called 
<B
CLASS="COMMAND"
>inetd</B
>;
	see the <I
CLASS="CITETITLE"
>Linux Network Administrators' Guide</I
>
	for more information.  </P
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